


Till The Cow-Cumdump Cums

by ruff_ethereal



Category: BanG Dream! Girl's Band Party! (Video Game)
Genre: Aged-Up Character(s), Alternate Universe - Modern with Magic, Breast Expansion, Butt Expansion, Costume Kink, Cow-themed Sex, Excessive Bodily Fluids, F/F, Futanari, Futanari Hikawa Sayo, Incest, Lactation Kink, Let's be clear this is extremely kink and depraved, Loss of Intelligence, Mind Manipulation, Sibling Incest, Wet & Messy
Language: English
Status: Completed
Published: 2021-01-08
Updated: 2021-01-08
Packaged: 2021-03-18 19:35:43
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,881
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28623426
Author URL: https://archiveofourown.org/users/ruff_ethereal/pseuds/ruff_ethereal
Summary: After Hina manages to get her hands on a modified slave collar that will transform her into a "brainless, cow-tittied cumdumpster" (in Moca's words), Hina insists she wear it and Sayo have sex with her. Despite her initial protests and good sense, Sayo agrees.It ends up being quite the mess.
Relationships: Hikawa Hina/Hikawa Sayo
Comments: 4
Kudos: 37





	Till The Cow-Cumdump Cums

It all starts with a raid.

About a dozen suspects are chained to the walls or the floors by magical currents of blue electricity, constantly pulsing, thrumming, and crackling violently if so much as a hair touched them. Sayo sighs as she dispels her lightning blade in a shower of sparks, she takes a moment to shake and flex her sword/casting hand.

Her fellow wardens swarm around her, making doubly sure that the suspects are all incapacitated, secured, and disarmed or beginning their investigations into whatever they were manufacturing here.

She hears a slow, drawn-out whistle, Sayo comes over to investigate.

"Looks like someone's been trying to sell to the really fucked up kinksters of the realm," Moca says as she looks over a crate full of metal collars.

“What are these?” Sayo asks as she peers at them.

"Slave collars but, extremely sinister kind," Moca drawls. "It's got all the standard features: geo-pulse tracking, biomonitor, voice-recognition connected to the air magic capacitors, plus material reinforcement features against wear and tear and forced removal. But, and here's the important part, it's also got some powerful body and mind modification spells."

"You can tell all that just from a glance, without touching them, or using any sort of instrument?" Sayo asked as she raised an eyebrow at Moca.

Moca chuckles and grins. “Moca is a genius, after all!”

Sayo suppressed a sigh. "How serious do you think the effects are?" she asked.

"Eh, assuming standard humanoid testing parameters, you're looking at supernaturally rapid manipulation, production, and suppression of hormones, neurotransmitters, and dermatological tissue," Moca replied. "This device comes with predefined parameters, so all the user needs to do is slap the collar on and their victim will be transformed into a brainless, cow-tittied cum-dumpster."

Sayo groaned and felt ill. “It’s never enough for them that they’re owning other sentient beings like property, is it?”

"Eh, you know what they say about the kinds of folks that would use these things," Moca said, shrugging.

“Please tell me you’re going to be destroying all this,” Sayo said.

“But of course!" Moca said, smiling. "Moca's going to have a lot of fun undoing all this work; whoever created the master-copy of this had one hell of an inscribing hand."

Sayo scowled. "Are you _seriously_ complimenting a criminal right now?"

"Moca can respect the product while denouncing the maker," Moca said. "That's just how you become a grand artificer."

Sayo shook her head and left; the reinforcements from the Tower were already coming in to secure the suspects and evidence, anyway.

She and the rest of the breach team were thanked then dismissed. The ill feeling followed her all the way home but she took comfort that all these evil tools were going to be scrapped, melted down, or dissolved back into mana.

Then a week later, she had one of them presented before her.

“Tada!” Hina said, beaming and smiling like usual.

Sayo screams, rockets out of her chair, and instinctively reaches for where her focus is usually holstered. "Hina, what the fuck do you think you're doing?!" she snaps.

Hina frowns, looking confused and troubled. “I’m showing you off my surprise, Big Sis?” she asks.

"Do you want to put a slave collar on me?!" Sayo screams, her heart racing.

"I don't!" Hina cried, horrified. "I would _never_ do something so awful to you, Big Sis, even if I was being forced to!" She's soon smiles and sunshine again as she continues "Instead, I'm going to put it on _for_ you!"

Sayo stares blankly at Hina's smiling, confident face, the anger and alarm from earlier near-instantly cool into exasperation. Sayo sighs as she grabs her chair and sits back down at their kitchen table.

She rests her elbows on it as she puts her hands together, looks at Hina, and flatly says, "Explain, now."

Hina nods. “So, I was talking to Moca the other day when she mentioned that she was struggling to get rid of these slave collars! You know, from that raid you went on last week?”

“So you knew exactly what these are and how we got them and now you want to put them on yourself?” Sayo asked, a mix of confused, exasperated, and concerned.

“Not as they were, Big Sis!” Hina said. “Moca’s already removed the nastier parts like the air magic capacitors so it’s not going to shock anyone _and_ they’ll can come off a lot easier now.”

“But what about the rest?” Sayo asked.

“That’s boppin’-est part!” Hina squealed, like she was about to get to the punchline of a hilarious story. “Moca couldn’t completely deactivate the enchantments _but_ she did manage to find a way to render their effects temporary, and redirected the excess power into reversal runes for extra insurance.

“Now, instead of being a horrible, awful, no-good device that’s going to ruin someone’s life, they’re just really zappin’ sex toys! I know you’re going to _love_ using me as _your_ brainless, cow-tittied cumdump” Hina said, giggling and winking.

A part of Sayo flared up in excitement, another part of her stamped it out in disgust. “No,” she said.

“Aww, but why, Big Sis…?!” Hina whines as she puts the modified collar back down. “Don’t you love it when my boobs lactate?” she said as she grabbed the sides of her breasts through her blouse and pushed them up. “I thought you’d enjoy adding a whole bunch of other, related kinks to it.”

Sayo blushes and looks away as her cock stirs in her pants. “Hina, this isn’t about what does or what doesn’t turn me on, it’s about _you.”_

She takes a deep breath before she turns back to her and continues, “I’m certain you understand this already but I need to say it out loud anyway: once you put this on, you won’t be able to take back your consent. You won’t even be able to alert me if something is going wrong, it may be too late before the time I finally notice and react.”

Hina frowns and looks at Sayo with a rare serious expression. “You say that like I don’t trust you with my life, Big Sis.”

Sayo frowns back at her. “The battlefield is a very different place from the bedroom, Hina.”

“Is it really?” Hina asks. “Emotions run high, instincts flare up, things can go wrong or right in an instant—only difference now is, the death’s the ‘little’ kind,” she said, smirking.

Sayo groans and shakes her head. “I don’t want to even consider this, Hina.”

“And I’m just saying that I would really, really like to do this with you because I think it would be super, duper boppin’ for the both of us,” Hina said before she shrugged. “But hey, you’re free to refuse—just know that the offer still stands.”

Sayo squeezes her eyes shut. “I’m leaving now, Hina,” she said as she got up, turned around, and left the kitchen.

“See you later, Big Sis!” Hina says, Sayo can just feel her radiant smile and enthusiastic wave.

Sayo debates going on a spontaneous vacation across several locations, places that even someone like Hina would have difficulty getting to in time to catch up to Sayo. Lisa has always been ragging her on to use her mandatory vacation days and generally remember to take it easy some times.

She ultimately decides that this doesn’t warrant such drastic measures. “I’m just going to erase this conversation from my mind,” Sayo thought.

And almost exactly two weeks later, they’re back in the kitchen, except this time, Sayo is in a bathrobe while as she sets wide, short metal milk tank with a large funnel on the kitchen table, while Hina is wearing a ridiculous “sexy cow” costume. It’s composed of a two-piece string bikini with a fake tail, long gloves and thigh-high stockings, and a cow-ear headband, all in black-and-white cow print. And of course, there is the crowning jewel of this depraved get-up, currently resting on the table.

“Did you really have to install a little cowbell on it?” Sayo said, putting a hand to her face as Hina picks up the modified slave collar.

“Of course I had to!” Hina says as she undoes the locking mechanism, now simpler and with an emergency release. “If we’re going to do kinky theme sex, we should go all the way! Though I’m still disappointed you said no to renting that barn.”

“Hay gets everywhere, Hina,” Sayo grumbles.

“Like _that’s_ ever stopped us before,” Hina says, rolling her eyes before she walks over to Sayo. In a softer voice, she asks, “Are you ready, Big Sis?”

Sayo sucks in a breath and lets it out through her nose. “I am. Are you?”

“I am now,” Hina says, giggling as she holds out the collar to Sayo.

Sayo hesitates for a moment before she takes it into her hands, Hina lowers her arms, closes her eyes, and lifts up her chin to expose her neck. Sayo looks at the serene, calm, trusting expression on Hina’s face, she steadies her hands as she delicately collars Hina.

_Click._

The sound and the sensation of it locking into place feel almost anti-climactic. Hina lowers her chin and opens her eyes, smiling and humming to herself as the runes engraved into the collar activate and flash for a moment, lines of mana surge into her body before fading away.

Sayo watches her carefully and warily, looking Hina up and down as she hums pleasantly, seemingly unaffected and unchanged by the collar—until suddenly, she is.

Hina’s eyes widen, Sayo swears she can almost see the intelligence in her eyes rapidly dim. Hina gasps as her expression grows mildly troubled, she’s soon blushing, panting, and drooling as her body begins to shift and change much more dramatically.

Her breasts rapidly swell into massive, rounded mounds, the bikini that was a perfect fit earlier comes immediately undone and becomes little more than a thin piece of thin fabric resting atop her engorged, exposed breasts. Her thighs grow wider and thicker, softer and squishier as well, no more traces left of the fit, toned muscles that Hina’s work and activities had effortlessly shaped over the years.

Because of the suddenness of the transformation, a side-effect of the magic actively impairing her brain, or how comically out of proportion her body was now, Hina stretches her arms out and flails, wobbling before she finally falls forward.

“Got you!” Sayo cries as she reaches out and catches Hina’s by her underarms.

Completely unintentionally, Hina’s engorged breasts squish against Sayo’s chest, Sayo feels her arms rubbing up against their sides, the soft, warm, pillowy flesh molds around them at even the slightest bit of pressure.

Sayo’s cheeks are already red from seeing Hina’s new body, they burn even hotter when Hina looks up at her and smiles. She can almost hear her cry out “Big Sis, look!”…

… But it never comes, Hina just continues smiling and staring at her.

Sayo stares deeply into those green eyes, concludes for certain that the lights are on but nobody is home. “By Lunaria, you don’t understand a thing that’s going on or what’s just happened to you, do you?” she asks.

Hina blinks then resumes staring at her, smiling, happy, unthinking.

The quiet voice in the back of Sayo’s head says, _“Wow, you can probably do_ _anything,_ anything _to her and she’d just go along with it_.”

Numerous ideas shove their way to the forefront of Sayo’s mind, some of them exciting, some horrifying, some a disturbing mixture of both. She shakes her head as she pulls Hina off of her and tries to steady her.

“We’re going to stick to the scene we discussed earlier, alright?” Sayo asks, looking sternly at Hina.

The effect is diminished by her whole face being flaming red, the beads of sweat trickling down her skin, and her shallow, heated breaths, but then again, Hina doesn’t care or seems capable of doing so.

Sayo sighs as she guides Hina to the table, turns her around, and perches her butt on the edge. The table groans as Sayo lets go and Hina shifts around and makes herself comfortable, Sayo worries that she should sit her down on a chair instead. That table had been through plenty but even the sturdiest of furniture had their limits.

Hina has no such worries—or really, anything going on in her mind, she just sits merrily with her thick, plush legs swinging underneath her, her hands in her lap, and her arms unintentionally squeezing her breasts together.

“ _Oh_ _, Celestina, they’re huge,”_ Sayo mutters as she finally gets a look at her chest.

She could grab them one of them with both hands and her arms and it still wouldn’t be enough. Each mound was likely large or larger than Hina’s head as they were, possibly even larger if the enchantments reacted with her innate stores of magic. And her pink, hard nipples were already leaking a thick streams of warm, creamy white breast milk, dripping down the generous curves of breasts before they dripped down to her thighs and a growing puddle on the floor.

A tent forms on the front of Sayo’s bathrobe, she feels terribly horny and terribly thirst in almost equal measure. She reaches for Hina’s breasts, she sinks her fingers in just before her areolae.

“Ahh~!”

Hina gasps with delight, milk briefly bursts out of both nipples. Sayo feels her fingers  sinking  into them, the flesh effortlessly melting and molding to her hands, and wonders, “How much milk will I get if I give her a good squeeze?”

“Kyaa~!”

Hina squeals and squirms as her nipple s turns into min i fire-hose s of milk,  torrents of milk explodes out and soaks the front of  Sayo’s bathrobe and  part  her face.

“Okay!” Sayo says as she quickly lets go, she closes one eye as milk almost drips into it. “That’s enough experimenting.” 

Hina pants, moans, and shudders, unquestionably, unmistakably turned on. Alongside the sweet smell of breast milk, Sayo can also detect the familiar scent of Hina’s arousal dripping down her thighs. Sayo check s, finds glistening fluid smeared all over her thick thighs as they near-constantly touch and rub against c onstantly touching and rubbing  against each other .

Sayo feels Hina grabbing her bathrobe, she looks up and nearly jumps back as she sees her expression: heated, hungry, desperate. Hina can’t form any words, just make shallow pants and whimpers, but that’s all Sayo needs to hear.

“I’m going to take care of you, Hina,” Sayo said. “But first, I’m going to try to keep us from making too much of a mess.”

It’s a struggle just to get Hina off the table. She won’t let go of Sayo’s bathrobe and accidentally pulls it open,  Sayo hears the table groan and its legs screech after  Hina stands up again,  and it’s an entirely new  struggle altogether just to get  Hina to  let  Sayo  go, turn around, and grip the edge of the table instead of  her .

Sayo’s  sweating and panting  as she finally gets Hina to bend over by the waist, place her leaking breasts over the funnel—or rather, squeeze them inside the cone, her breasts were far too large.

“Agh, we should have just started from this position in the first place,” Sayo grumbled to herself.

She was far thirstier than she was horny now, she habitually made for the fridge when she caught Hina looking at her expectantly. Sayo paused, looked at  Hina’s face, down at her engorged breasts  stuffed into the funnel and shoving the milk tank around at even the slightest movements .

“Don’t hesitate to prioritize readily available resources,” Sayo thought as she grabbed one breast with both hands.

Hina squealed in delight,  she squirmed and wiggled happily as Sayo pulled it out of the funnel. Already,  Sayo could hear so much milk  pouring down the funnel and into the milk tank, a fresh mess splattering and spilling all over the surface of the table.  Sayo pulled Hina’s nipple to her, saw the  milk spilling out from  it like faucet and started having second thoughts. 

Hina looked  at her and whimpered, a pathetic expression on her face . 

Sayo hesitated before latched onto Hina’s nipple and started sucking.

As _so much_ milk burst into her mouth and rushed down Sayo’s throat, she recalled that time she tried to drink directly from a soda fountain nozzle and learned what “Too much of a good thing.” meant. Hina’s breast milk didn’t make her sinuses burn but she still pulled away coughing, spitting, and hacking, milk dribbling out of her mouth and from her nose.

Sayo recovered wiped herself off some, she noticed Hina moaning and shaking her chest and her butt, clearly begging for more.

Sayo scowled. “You can’t even think about--” she sneezed. She stopped, wiped herself up some with her bathrobe, before she finally shrugged it off. “You can’t even think about anyone other than yourself, can you, you dumb cow?” she said as she walked around to Hina’s plush, soft, and _massive_ ass.

Hina looked over her shoulder and tried to follow Sayo as best as she could, her eyes still dim, her cheeks flushed red, her mouth and tongue shiny and slick with drool.

“Well, I’ve had enough of trying to please _you!”_ Sayo cries.

_Smack!_

She spanks Hina on one asscheek, Hina cries out and shifts forward, rocking the table and dragging the milk tank  along with her . Sayo pauses, taken aback by how soft the impact felt, mesmerized by all that bountiful flesh rippling and jiggling for several seconds after.

Sayo shook her head and put the stern expression back on as she reached between Hina’s ass cheeks, hooked her Hina’s bikini bottoms, and pulled!

… But already thin then stretched even more so by Hina’s expanded ass, it snaps off immediately and with a pathetic sound, Hina doesn’t notice or feel a thing.

Sayo sighs as she tosses it to the side. “We really need to account for just how much your body’s going to expand if we’re going to play with costumes  and body modification magic like this...” she mutters to herself before  grips  one  ass  cheek and pulls it to the side, her other hand wraps around her erect cock.

“It’s your turn to please _me_ , Hina!” Sayo snapped as she guided her shaft to Hina’s pussy, pressed her cockhead against it.

Hina gasped as Sayo began to penetrate her, she looked shocked before she starts shamelessly panting and drooling again.

“Fuck, you’re so wet,” Sayo muttered.

Hina’s pussy was soaked—gushing, actually, fresh arousal pouring out to meet Sayo’s  cock  as she  sinks in.  Hina’s  inner  walls eagerly spasm and tighten around her girth, Sayo  gasps her  plush, soft,  and slick thighs wrap  and mold  around  her  cock as well.

Sayo makes an experimental thrust, Hina’s whole body seems to rock  from it ,  she cries out in pleasure. Sayo hears so  much of her milk gushing out of her breasts, feels her arousal  gushing out, making Sayo’s cock, her pussy, and her thighs all the slicker and messier.

“Did the enchantments make her extremely sensitive?” Sayo thought as she began to speed up. “Or maybe it’s just because she’s getting fucked by her Big Sister,” she thinks, a shaky smile spreading on her face.

Sayo’s hips noisily slap against Hina’s massive ass as she fucks her, lewd, wet noises fill the air as so much of their pre-cum and arousal spill out and get smeared around. Hina’s breasts and ass cheeks near-constantly jiggled and bounce, the table rocks and creaks, the milk tank quietly screeches as Hina drags it around, the breast milk inside it noisily sloshed around, the ones outside loudly spilled, splattered, and dripped onto the growing mess all around the kitchen. 

Sayo’s head feels cloudy, her body feels like it’s on fire, her feet step and slide all over the  over the wet  floor  as she clumsily pounds Hina . She slips and falls on top of  her , her body feels so soft and plush as she  lands; Sayo’s hands  instinctively try to find something to grip on, find the sides of Hina’s breasts, and clutch  _ hard. _

Hina screams in pleasure as she arches her back, ludicrous amounts of milk explode out of her breasts and keep on gushing, the surface the table is soaked thick in it. Sayo hears it spilling all over the table and the floor, bubbling up and overflowing from the milk tank and the funnel--

\--And then, because of all the weight that had been steadily sliding to one side of the table, it finally tips over and sends them crashing to the floor.

The  accident is  loud, messy, and chaotic  as the  metal milk tank  and the funnel bang and clang as they tumble and roll around the floor, the edge of the wooden table slams against the floor , Hina and Sayo  yell  as they find  themselves suddenly thrown around.

By the time things calm down, the milk tank has come to a stop on its, the table’s still in piece as it lays on one side, Sayo is still on top of Hina as she lays in a giant puddle of milk.

Worry  floods Sayo  and instantly kills her orgasm and her erection. She  gasps and w hispers, “Hina? Hina! Hina, are you alright?!”

Hina just murmurs and moans in a daze. Sayo pulls out, climbs off of her, and starts examining her.

Nothing is bent the wrong way and she’s resting  comfortably on top of her engorged chest.  “Should have realized you’ve already got built-in padding...”  Sayo sighs as she reaches for the collar, feels around until  she hits the emergency release.

_Ka-chik._

The collar comes off instantly, Sayo pulls it off of Hina and tosses it away before she sits down on the flood of milk on the floor, her legs splayed out before her.

It takes a few minutes for the effects to start wearing off but once they do, they do so in a hurry. The light of intelligence gradually returns to Hina’s eyes, she starts to carefully push herself off the floor, her thighs and breasts steadily shrinking back to normal size as she crawls over to Sayo.

Hina stares at Sayo’s frustrated expression and mutters, “Big Sis…?”

Her words are slow, careful, like she’s struggling to remember how to say them alongside what to say in general.

“I’m fine,” Sayo grumbles. “Are you?”

It takes Hina a moment to process it before she smiles and says, “Yes!”

Sayo sighs, puts her palms on the floor, then leans back. “We are never doing this again...” she mutters as she stares up at the ceiling.

“Aww, was fun!” Hina says as she crawls over to Sayo and looks down at her face.

Sayo scowls at her. “For _you,_ maybe. You’re cleaning up this entire mess, by the way, this has been far, far more effort than I expected to put and it wasn’t nearly worth it!”

Hina giggles and says, “Okay!”

She gives Sayo a quick kiss before she crawls off in the direction of the supplies closet.

Sayo blushes, scowls, and turns away from Hina.

**Author's Note:**

> I have thoughts of adding one of my other unrealistic, breast-related kinks here but I think this is enough depravity for now.


End file.
